Fetch node configuration through node custom module

Summary:
Grains are only defined in the salt unit,
ie after it's needed first in network.

Instead, we can fetch information directly from pillar.

This fixes the following issue:
Rendering SLS 'base:roles/core.network.gre' failed:
Jinja variable ... has no attribute 'roles' ; line 11

This is a follow-up for 050d063bbefe.

Test Plan: Deploy roles/core on router-002

Reviewers: yousra, Duranzed

Reviewed By: Duranzed

Differential Revision: https://devcentral.nasqueron.org/D3938